The National Capital Authority, the federal agency which controls the Deakin and Forrest residential precinct, was withdrawing its support for their plan to build four disability-accessible townhouses on the block.

The next morning, the authority released a draft amendment to its National Capital Plan that would limit development in the area to just one dwelling per block, ending a long-standing trend towards greater density.

"We were gobsmacked, they'd told us every step of the way it was fine, we were down to the finer details like adjusting fence heights and then this came out," Ms Knowles said.

"It was a total back flip. We're just devastated."

The pair have now joined a group of about 20 others who say the new rule will shave thousands off their property values, disrupt their plans to down-size in older age, and roll back the ACT government's vision of diversifying housing options, instead leading to "yet more McMansions" in the area.

Many are now considering packing up and selling. Jim Bain bought his Deakin home more than 20 years ago for the future prospects of the 1700 square metre block. While he estimates it is now worth between $1.5 to $2 million dollars, he believes the amendment will drop the value by about 30 per cent.

"Now the family's moved on...our longer-term plan was to put dual-occupancy or triple occupancy on the block which has been allowed by the NCA for many years," Mr Bain said.

An NCA spokeswoman said the authority had "carefully considered the changing nature of the precinct" and developed a view that, in order to uphold its national significance and 'Garden City' character, development should be restricted to a single dwelling per block.

The only exception to this new rule would be blocks facing State Circle, where townhouses have already been built.

While a number of embassies and diplomatic residences fall within the area affected, the spokeswoman said no changes were proposed to diplomatic estates.

"The NCA supports greater density and urban consolidation in the appropriate areas," she said, noting the agency's hand in the New Acton Precinct and Constitution Avenue.

But some locals have questioned why an issues paper released by the authority last year, which residents and developers were told to follow in their planning considerations, made no mention of the looming single dwelling amendment.

The paper acknowledged the precinct was largely characterised by single homes, but it also noted that its location near "major employment areas, the city...and transport routes" and "changes in population and lifestyle" had resulted in demand for alternative dwellings.

"This diversity of housing types enables people to remain in their community in housing that best suits their needs," it said.

In his email to Ms Knowles, NCA chair Terry Weber said "feedback from [that paper] helped inform the Board's considerations".

The "cumulative impacts" of more multi-dwelling units such as duplexes, townhouses and apartments in the area meant the character of the precinct was now at risk 'without changes to current planning policy", he said.

The NCA spokeswoman said such concerns were reflected in public submissions to the paper, where a number of people suggested the one dwelling rule.

Out of 37 submissions published alongside the draft amendment, more people favoured increased density than opposed it. But, on both sides, a majority agreed the "garden city" feel of the neighbourhood should be preserved.

Architect Tony Trobe said a number of his clients would be affected by the single dwelling rule. The precinct was unique, he said, as the only residential area of its kind under the control of the Commonwealth agency, but the amendment seemed to run counter to a long-standing move towards increased density led by the former head of the authority, Malcolm Snow.

Planning spokeswoman for the ACT Greens Caroline Le Couteur agreed the character - and canopy - of Canberra's neighbourhoods ought to be protected but said this did not have to come at the cost of diverse housing options.

"This can be done by careful changes to the planning rules to encourage a diverse mix of housing while limiting the amount of each block that can be built over, protecting our existing trees, and protecting more green space on each block," Ms Le Couteur said.

"Building smaller homes while protecting our vegetation will keep our city sustainable, and help to mitigate the effects of climate change."

The NCA said it welcomed public submissions on the draft amendment until April 16.

A spokeswoman for ACT Planning Minister Mick Gentleman confirmed the government was preparing a response.

with Han Nguyen